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Abstract

The ability to model the CARS (coherent anti-Stokes Raman spectroscopy) spectra of combustion molecules is an important part of the process of extracting diagnostic information from CARS signals; measurements of temperature or species concentration usually require that the experimental signatures be analyzed with the aid of computer-synthesized spectra. This dependence on theoretical models arises from spectral interference effects unique to CARS; because it is a nonlinear optical process whose signal strength depends on the squared modulus of the third-order electric susceptibility, the spectral shape is affected by interferences between neighboring Raman transitions and between the resonant and nonresonant contributions to the susceptibility. This paper will review the status of modeling the spectra of molecules important in combustion, such as N2, H2O, and CO2. It will be shown that accurate modeling generally requires highly precise knowledge of line positions and reasonable estimates of Raman linewidths, and the sources of these data will be discussed. In many practical applications, pressures well above atmo-spheric are encountered, and the phenomenon of collisional narrowing can become important. This effect, which has its origins in rotationally inelastic energy transfer between adjacent J-states, gives rise to a pronounced narrowing of signatures with increasing pressure. The status of modeling this effect in N2, H2O, and CO2 will be described, and it will be seen that good agreement with experiment can be achieved using either the Gordon rotational diffusion model or phenomenological models for inelastic energy transfer rates.

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Journal of Applied Remote SensingJournal of Astronomical Telescopes Instruments and SystemsJournal of Biomedical OpticsJournal of Electronic ImagingJournal of Medical ImagingJournal of Micro/Nanolithography, MEMS, and MOEMSJournal of NanophotonicsJournal of Photonics for EnergyNeurophotonicsOptical EngineeringSPIE Reviews